Dewa Sanzan

Mt. Haguro is accessible all year round, but Mt. Gassan and Mt. Yudono close during winter because of snow. The (short) peak climbing season lasts from July to mid-October. Festivals are held to celebrate the opening and closing of the shrines.

Hagurosan is the most easily accessible of the three sites and the only one that can be visited without some hiking.

• The well-marked trail from the base of Hagurosan winds its way through the forest for a while, passing the beautiful wooden Gojū-no-tō (五重塔 Five-Story Pagoda) along the way. Built some 600 years ago, the pagoda is registered as a National Treasure.
• Soon the ascent of 2446 stone steps starts. Easy to walk and not particularly steep, the climb is tiring in its sheer relentlessness, but you can pause at a popular teahouse halfway up for refreshments and get a certificate of climbing up the hard way. This will take around one hour if you're in reasonably good shape and don't dally too much. You can also visit the place where Japanese Poet Bashō wrote one of his many famous poems. Turn right at the cross after the tea house (not up the steps). You will have to backtrack after visiting.
• Mt. Haguro (a mere 414 m) does not have an identifiable peak as such, at one point the trail simply flattens out and after a torī gate you will find yourself on the grounds of the shrine. The main attraction here is the Sanzan Gōsaiden (三山合祭殿), venerating the spirits of all three mountains.

The hike to the top of Mt. Gassan and across to Mt. Yudono is a relaxed full-day hike, thought it should be approached with due respect. Obtain a map of the area from any nearby tourist office before you set out and take heed of weather conditions. With a forecast of rain and without proper rain gear, hiking to the peak can be not only extremely uncomfortable (you will get soaked) but dangerous as well with high speed wind gusts, slippery rocks, and very cool temperatures near the top.

• Buses deposit passengers at Gassan Eighth Station (月山八合目, Gassan Hachigōme, ~1400 m), from where it's a fairly easy two and a half-hour hike 500m vertically up the mountain through rolling plains often shrouded in mist.
• The peak of Gassan (月山頂上, 1984m) features the simple Gassan Shrine (月山神社). Entry costs ¥500 but includes quick purification by the resident Shintō priest.
• If you continue the trail across the peak and down, keeping to the right when the trail splits, you will after 45 minutes encounter a last split where one trail continues up to the peak of Mt. Yudono (~1500 m).
• The real fun starts on the other trail to the right, which climbs down to Mt. Yudono Shrine (湯殿山神社 yudonosan jinja), which is considerably steeper and in places equipped with steel ladders to climb down, though most of these have been rebuilt. The time down from the trail split is one hour 20 minutes.
• The reward for your efforts is Mt. Yudono Shrine (湯殿山神社 yudonosan jinja), the holiest shrine of the three. Photography is prohibited and so is telling outsiders what you have witnessed - so Wikivoyage shall not spoil the surprise!
• The Mt. Yudono parking area and bus stop with a variety or eateries is only a short walk from the shrine.
• Buses departing Mt. Gassan and Mt. Yudono are few and far between, so plan your day accordingly. The last bus back to Tsuruoka train station leaves Mt. Gassan at 16:00 and Mt. Yudono at 16:30.

You may want to stop by bus at Ōami and visit the Dainichibō Temple (大日坊) or the Chūrenji Temple (注連寺). To get there, take the Mt. Yudono leg bus and get off at Ōami. Unfortunately, bus schedules may not mesh well with various interesting temple tours.

• 🌍 Dainichibō Temple (大日坊), Ōami, Nyūdō−11 (backtrack to the street before the bus stop -- or, if you are at the bus stop, go to the street right of the vending machines -- and continue uphill for about 5 minutes; the temple entrance is a big gate). Offers a priest's remains and various other interesting trophies. English is not well spoken and no pamphlet is available. However, everybody will be nice and comforting while showing you around. 
• 🌍 Chūrenji Temple (注連寺), Nakadai-92-1 Ōami (continue the street following the bus, turn right at the big intersection and walk for about 20 minutes into the forest road to get to Chūrenji). Offers another Buddhist priest who starved himself to death while praying. Though this one is said to have been a criminal before becoming a priest.